You just need to cut the flower stem down to about 2 inches from the top of the bulb but keep the leaves. Continue to look after the plant all summer to encourage further leaf growth and give it some general fertiliser at least once during that time. Then at the end of the summer the leaves will begin to yellow and die down naturally.
When they turn yellow, cut them back to 2 inches above the neck of the bulb and then remove the bulb, clean it and dry it out. Leave it in a cool, dark place such as your garage until the end of the year then re-plant it.